NOx formation in lean premixed noncatalytic andcatalytically stabilized combustion of propane

NOx formation in catalytically stabilized, and noncatalytic combustion of lean, premixed, atmospheric, propane/air mixtures is experimentally and numerically investigated. For this, the combustion of preheated (400°C) mixtures with adiabatic flame temperatures from 1300 to 1500°C is stabilized by catalytically active and inactive ceramic foam structures in an adiabatic combustion chamber. To study catalytically stabilized combustion, a fraction of the fuel is converted catalytically within the active foam. In noncatalytic combustion, no fuel is converted within the inactive foam. In both cases, a complete burnout is accomplished downstream of the foams in a homogeneous combustion zone. For the modelling, the homogeneous combustion is represented by a one-dimensional, adiabatic, premixed, laminar flame. A detailed chemical mechanism for propane combustion and NOx formation is included. Experimental and numerical results for NOx emissions are presented, and the channels of NOx formationare identified. The numerically obtained NOx emissions compare well with the experimental data. The calculations show that only little thermal NOx is formed in the burnt gas by the Zeldovich mechanism because of the low flame temperatures. The NOx emitted is mainly prompt-NO formed within the combustion zone by the Fenimore, the Zeldovich, and the N2O mechanisms. Under the conditions above, the formation of NO reacts most sensitively with respect to the reaction rates of the reactions CH+N2→ HCN+N and CH+O2→O+CHO. The NO emissions of the catalytically stabilized flames are remarkably lower than those of the noncatalyticflames and depend on the fraction of catalytically converted fuel. The higher this fraction is, the lower the NOx emissions are. The calculations indicate that the reduction of prompt-NO formation in catalytically stabilized combustion is due to a decrease in the NO formation via the Fenimore and the N2O mechanisms, whereas NO formation via the Zeldovich mechanism is scarcely influenced.